Carbon dioxide is responsible for only about half of the Earth's greenhouse effect warming. Methane, which is increased by escaping natural gas,
contributes 18%, and nitrogen oxides, which are products of fossil fuel
burning, account for 6%. Another major contributor is chlorinated fluorocarbons (CFCs), which are responsible for 14%. These are familiar as the
working material in air conditioners, refrigerators, and freezers, and as the
propellant in spray cans. (Other important sources of methane include
flooded rice paddies and cows burping up gas from their stomachs.)

Another cause of the greenhouse effect is cutting down forests, since
plants take carbon dioxide out of the air. Clearing of the Amazon rainforest
is believed to be especially serious. Worldwide, an area of forests equal to

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